1. Senior Structural Engineer, AIT Consulting, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand.
2. Professor, Dept. of Civil and Environmental System Engineering, Konkuk Univ., 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea (corresponding author).
3. Professor, International Center for Numerical Methods in Engineering (CIMNE), Universidad Politecnica de Cataluna, Edificio C1 Campus Nord, 08032 Barcelona, Spain.